In telecommunication satellites, it is necessary to amplify radio frequency signals received in order to retransmit them, via transmit antennas, to subscribers, that is to say receiving stations, for example situated on the ground or in airborne carriers. The devices embedded in the satellites making it possible to retransmit the radio frequency signals are commonly called “repeaters”. A repeater comprises communication subsystems or channels and each communication subsystem can typically comprise, in its output section or “downlink channel”, one or more travelling wave tube amplifiers, linearized or not, commonly designated (L)TWTA, which stands for “(linearized) travelling wave tube amplifier”, the travelling wave tubes commonly being referred to by their acronym TWT. For optimum performance, a TWT is usually used close to its saturation region, and constitutes an equipment item that is particularly sensitive to input levels above its saturation power. Excessively high input levels can in fact result in deterioration, even destruction, of the TWT. An application of input levels above an acceptable threshold is commonly referred to by the term “overdrive”.
According to known techniques, an amplification module of channel amplifier type can be arranged upstream of the LTWTA, making it possible to condition the radio frequency signal applied as input to the TWT, notably via a pre-amplification. Known channel amplifiers are, for example, called CAMP, which stands for “Channel AMPlifier”, or even “DLA”, which stands for “Driver Limiter Amplifier”. Typically, a linearized travelling wave tube amplifier LTWTA benefits from an output power limiting function that makes it possible to protect the TWT when the levels of the input signals are too high. This function is, as is known, handled by a limiter situated upstream of the TWT and downstream of the predistortion means used for the linearization of the TWT, and based on a technology of MMIC type, MMIC standing for “Monolithic Microwave Integrated Circuit”, based on a material of semiconductor type, for example gallium arsenide AsGa. Such a solution, well known and described notably in the document U.S. Pat. No. 4,701,717, does, however, present two major drawbacks, explained hereinbelow.
The first drawback is linked to the robustness of the technology employed, which cannot be engaged over indeterminate overdrive periods. The engagement in terms of duration is limited, and depends on the overdrive level considered, and on the knowledge acquired by experiencing real conditions. Fatigue tests, or life tests, can be carried out, but the engagement in terms of duration must necessarily be bounded by the duration of the life test, since there is no accelerated ageing process currently in existence.
The second drawback is linked to the fact that the gain control of the channel amplifier is all carried out upstream of the limiter, which means that all of the communication subsystem of the repeater can only be protected when the TWT is operated close to saturation. In practice, operating the TWT with a small signal is generally done by reducing the gain of the channel amplifier situated upstream of the LTWTA. In this case, the output limiter is operated in its linear region and no longer close to its saturation. It is therefore no longer able to clip the peaks of the interfering signals, whether deliberate or not, this phenomenon being able to create a significant degradation of the link budget and therefore of the system performance levels.
Another known limiting solution consists in using a power regulation loop with threshold as described in the document U.S. Pat. No. 4,701,717, but this solution exhibits an excessively long response time that is incompatible with the filtering of pulsed signals originating from a scrambler or from an overdrive of a duration less than this response time.
Thus, there is at this time no known solution that makes it possible to protect a linearized travelling wave tube whatever its operating point, of strong back-off or at saturation, in the case of a scrambler or permanent overdrive, whether it is continuous or pulsed. It is therefore not possible to ensure a minimum service level, that is to say satisfy minimum communication requirements, in the case of the presence of a scrambler or permanent overdrive.